The present invention relates to a scroll fluid machine suitable for use as an air compressor, a vacuum pump, etc.
In general, a scroll fluid machine includes a fixed scroll member and an orbiting scroll member that are disposed to face each other. The fixed and orbiting scroll members each have an end plate and a spiral wrap portion standing on the bottom surface of the end plate. The wrap portions of the two scroll members, which are disposed to face each other, overlap each other to define a plurality of compression chambers therebetween. In this state, the orbiting scroll member is driven to perform an orbiting motion with respect to the fixed scroll member, thereby successively contracting the compression chambers to compress a fluid, e.g. air.
When compressed air is produced, the pressure of compressed air may apply an excessive thrust load to the orbiting scroll member in the direction of the rotating shaft. To minimize the thrust load, a related art proposes a structure in which a back-pressure chamber is provided at the back of the orbiting scroll member so that a part of the compressed air is introduced into the back-pressure chamber, thereby reducing the thrust load by the pressure created in the back-pressure chamber (for example, see Japanese Patent Application Publication No. 2004-28033). In this structure, the orbiting scroll member is coupled through an orbiting bearing to a rotating shaft having a crank portion, and auxiliary cranks are provided between the fixed scroll member and the orbiting scroll member to prevent the orbiting scroll member from rotating around its own axis.
In the above-described related art (Japanese Patent Application Publication No. 2004-28033), the orbiting bearing is provided on the center of the back of the orbiting scroll member. Therefore, the pressure-receiving area of the back-pressure chamber is a doughnut-shape area around the outer periphery of the orbiting bearing. Accordingly, it is unavoidably necessary in order to obtain a sufficiently large pressure-receiving area to increase the size of the orbiting scroll member radially outward by an amount corresponding to the area occupied by the orbiting bearing. Thus, the overall size of the compressor tends to increase.
Further, in the related art, the back-pressure chamber has a doughnut shape in diametrical cross-section because the orbiting bearing is provided on the center of the back of the orbiting scroll member. Therefore, two seal members different from each other in diametrical size are needed to hermetically seal the back-pressure chamber. Consequently, the sealing performance degrades even when only one of the two seal members has become worn. Thus, reliability is likely to decrease.
In addition, in the related art, the bearings of the auxiliary cranks, which constitute a rotation preventing mechanism, are provided between the orbiting scroll member and the fixed scroll member. Therefore, the bearings of the auxiliary cranks need to be disposed radially outward of the wrap portions of the orbiting and fixed scroll members. This causes the compressor to increase in size diametrically.